Means for measuring the delay angle in a static converter

ABSTRACT

For measuring the delay angle in a static converter, a rampfunction generator is provided for each rectifier phase connected to the alternating current network. The ramp-function generator includes a timing device responsive to passage through zero of the commutation voltage derived from the network, and a connecting member which connects the ramp-function generator to a sensing and holding circuit. The connecting member is controlled by the control pulse device of the converter so that the signal stored in the sensing and holding circuit at any moment corresponds to the delay angle for the rectifier last ignited.

United States Patent 1191 Liss 1 Apr. 23, 1974 [5 MEANS FOR MEASURINGTHE DELAY 3,704,408 11/1972 Schroeder 3211s ANGLE IN A STATIC CONVERTER3,766,465 10/1973 Reeve 321/5 [75] Inventor: Giite Liss, Ludvika, SwedenI Primary Exammer-W11l1am H. Beha, Jr. Asslgneei Anmlnna svenskaElektl'lska Attorney, Agent, or Firm-Jennings Bailey, Jr.

Aktiebolaget, Vasteras, Sweden [22] Filed: May 2, 1973 571 ABSTRACT [21]Appl' 356488 For measuring the delay angle in a static converter, aramp-function generator is provided for each rectifier [30] ForeignApplication Priority Data phase connected to the alternating currentnetwork. May 3, 1972 Sweden 5788/72 The ramp'functicn gelerator includesa timing device responsive to passage through zero of the commuta- 52U.S. c1 321/40, 321/27 R 321/38 "(mags derived frm the "etwmk, and a [51] Int. Cl. H0 2m 7/46 ing member which connects the ramp'funcfion gene[58] Field of Search u 321/4 5 13 27 R 38 ator to a sensing and holdingcircuit. The connecting 321/40 member is controlled by the control pulsedevice of the converter so that the signal stored in the sensing [56]References Cited and holding circuit at any moment corresponds to theUNITED STATES PATENTS delay angle for the rectifier last ignited.

3,551,778 12/1970 Ekstrom 321 /5 3 Claims, 4 Drawing Figures i I- i s/ ll Uref. l Ur/ R4 '1 l 1 I H 1 1 1 l 11 R7 54' 1 1 1 1 1 1 Un? 5 I G9]: IU

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BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a means for measuring the delay angle of therectifiers in a static converter connected to an alternating currentnetwork.

2. The Prior Art The invention thus relates to a so-callednetworkcommutating converter in which the commutation voltage for therectifiers is derived from the alternating current network. Therectifiers in the converter are controlled by a control system having adelay angle determined either by a regulator for current, power or thelike, or by a limit value transducer, for example a commutation margincontrol device.

The direct voltage arising in the converter is defined by the expressionE.cosz where E is the value of the alternating voltage connected to therectifiers and a is the delay angle. In order to limit the reactivepower in the converter it is desirable in rectifier or inverteroperation to keep a as close to 0 or 180 as is practically possible andconvenient and in order to achieve this at a desired direct voltagevalue, it is usual to provide the converter transformer with a tapchanger to vary the voltage E depending on the real value of the delayangle a.

However, in more modern control systems oscillators are often used whichemit control pulses to the rectifiers with a certain frequency. Thus,the signal from the control system becomes merely an order to alter thedelay angle in one direction or the other whereas, during stationaryoperation, this signal becomes zero and the value of the delay angle issubstantially unknown.

SUMMARY OF THE INVENTION The present invention relates to a relativelysimple device for measuring the delay angle and the measurement takesplace from the zero passages of the commutation voltages and the controlpulses of the rectifier and the device is constituted by a ramp-functiongenerator for each rectifier phase of the current converter whichincludes a timing device responsive to the passage through zero of thecommutation voltage and a sensing and holding circuit connected to therampfunction generator by a connecting member which is controlled by thecontrol pulse device of the converter so that the signal stored in thesensing and holding circuit at any moment corresponds to the delay anglefor the rectifier last ignited. According to the invention a newmeasuring value for the delay angle is obtained immediately after eachcommutation so that the device is as rapid as possible in operation.

It is also important to be able to sense the delay angles when there areseveral cooperating converters in a converter station, controlledaccording to the oscillator principle. The regulator can then givecollected orders to the various converters without it being at all clearhow the orders are distributed between or carried out by the variousconvertersBy measuring the delay angles, the invention makes it possibleto counterbalance the deviations in the delay'angles, and thus the powerdistribution over the various converters.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further describedwith reference to the accompanying drawings in which FIG. 1 shows aconverter of conventional design,

whereas FIG. 2 shows a device according to the invention.

FIG. 3 shows the various voltages which arise in such a device.

FIG. 4 shows how the device according to the invention can be used tobalance a number of cooperating converters in relation to each other.

DESCIPT ION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a converter havinga rectifier bridge with six rectifiers 1 6 and a converter-transformer7. The converter is connected to an alternating current network by phaseconductors RST and to DC busbars 9, 10. The rectifiers 1 6 arecontrolled by a control device 8 of conventional type, which emitscontrol signals g g; to the rectifiers.

The control device 8 is of the oscillator type, for example according toUS. Pat. No. 3,551,778, which, depending on the frequency of the ACnetwork, delivers control pulses the phase position of which can bealtered in one direction or the other by a regulator without thephaseangle, that is the delay angle, of the control pulses everappearing as an available parameter or magnitude in the control system.

The means for measuring the delay angles reached by the control signalsg g, is shown more clearly in FIG. 2 and its function-will be explainedwith reference to FIG. 3.

To the left in FIG. 2 are three ramp-function generators ll, 12, 13which derive three saw-tooth voltages U U and U from the threecommutation voltages, that is the main voltages U U U between the phaseconductors R, S, T in FIG. 1. The first of these saw-tooth voltages isshown in FIG. 3.

The ramp-function generator consists of an amplifier A1 with capacitivefeedback C1, the amplifier thus operating as an integrator. A supplyvoltage U is connected to the amplifier input over a resistor R1. Thevoltage U may be a constant voltage or it can be made proportional tothe frequency of the AC network, the delay angle then being measured inelectrical degrees instead of pure time.

The amplifier Al is parallel-connected with a transistor Tl controlledby a monostable flip-flop device Ml which is controlled by thecommutation voltage U in such a way that the device switches over andshortcircuits A1 over T1 when the commutation voltage passes throughzero. The output voltage U from Al thus acquires the saw-tooth shapeshown in FIG. 3 with a period of a length equal to half a period of thecommutation voltage. The righthand side of FIG. 2 forms sensing andholding circuits for the angle measuring device.

The output from 11 is connected over a resistor R4 and a contact 81 toan amplifier A4 of the voltagefollower type. A capacitor C4 is connectedover a resistor R7 to the input on A4 and over the contact S4 to theoutput on A4. The contacts S1 and S4 are both controlled by the signal Ufrom a flip-flop device which is in turn controlled by the short pulsesg and g generated when the rectifiers 1 and 4 fire. These signals areindicated in the third row in FIG. 3. This means that, when one of therectifiers 1 and 4 fires, the contacts S1 and S4 close briefly,whereupon the capacitor C4 is charged up to the actual output voltagefrom A1. This voltage is an unequivocal measurement of the correspondingdelay angle. More specifically, in the curve U,, in FIG. 3, the abscissaconstitutes the phase angle while the ordinate is the output voltagefrom A1. When the pulses g, or g, in FIG. 3 have ceased and the contactsS1 and S t have opened, the output signal Ua from A4 will correspond tothe charging voltage U... in the capacitor C4 and will therefore be ameasurement of the latest delay angle, that is for the rectifier mostrecently fired.

For the two other rectifier phases 3, 6 and 5, 2 in FIG. 1 correspondingramp-function generators 12 and 13 are connected to the amplifier A4over contacts S2 and S3 controlled by flip-flop devices M and M6controlled by the control pulses g g and g g respectively. The contactS4 is affected by all the signals U U U so that the delay angles fromeach of the rectifiers l 6 in FIG. 1 are recorded in the capacitor C4and the amplifier A4. The output voltage U thus acquires the step-shapeshown in the fourth row in FIG. 3, the length of the step correspondingto the conducting interval for the corresponding rectifier whereas theheight indicates the delay angle.

If the delay angle for a single rectifier l 6 is to be recorded, thiscan be done by simply disconnecting the control pulses g in FIG. 2 fromthe other rectifiers so that only the control pulses from the rectifierunder observation will reach the corresponding flip-flop device M4, M5and M6.

By supplying the signal from A4 in FIG. 2 to a capacitive circuit, thesignal U will come to represent an average value for a number ofsucessive delay angles. Another possibility is to record the maximum orminimum value of the delay angle over a certain period.

The device according to FIG. 2 can be used to control a tap changer orpurely for inspection purposes to see how the converter is operating orit may be included in the control arrangements to balance twoseries-connected converters in relation to each other, as shown in FIG.4.

As mentioned earlier, no exact measure of the relevant delay angleexists in the case of oscillator control of converters and if, forexample, there are two seriesconnected converters in a converterstation, controlled by a common regulator, the two converters may verywell operate with widely differing delay angles so that theseconverters, carrying the same current, operate with completely differentdirect voltages and thus different loads. With only two converters it ispossible to compare the phase positions of the control pulses of the twoconverters and if necessary adjust the control pulses in relation toeach other. However, with several converters, such a pairwise comparisonbecomes rather complicated and it is therefore proposed to obtain theaverage value of the delay angles measured by the device shown in FIG.2.

FIG. 1 also shows converters 14 16 seriesconnected to that describedfirst and similar to it. The lower end of such a converter station maybe connected to earth whereas the upper end is connected by theconductor 9 to a power transmission line. A measuring transducer 17 isconnected in the earth connection to measure the direct current in theconverters.

FIG. 4 shows how this measuring transducer is connected to a currentcontrol amplifier 18, common for the converters. A desired value for thedirect current can be obtained from a desired-value emitter symbolicallyindicated by a potentiometer 21. The output signal from the controlamplifier is supplied to a control system 19 with a control pulsegenerator 8 for each of the converters, also shown in FIG. 1.

The signals from 8 which are supplied to the rectifiers in the relevantconverter are also supplied to a control angle measuring deviceaccording to FIG. 2 so that voltages U 1 4 are obtained as a measure ofthe delay angle in the various converters. The average value of thesedelay angles is then calculated in the amplifier A5 and compared for theindividual converter in the amplifier A6 with the delay angle of theconverter itself. Any deviation between the delay angle of the converterin question and said such average value is added in the form of an extrasignal to the signal from the control amplifier 18 over a summationmember 20, the delay angle of the converter then being adjusted to theaverage value for all the converters.

In this way all the converters will very quickly operate with the samedelay angle and, even if there is an alteration in the delay anglecaused by a control action in the control system, the delay angles willfollow without any delay.

Because the delay angle itself is being measured all the time, it isirrelevant whether the converters operate in the same phase position oras is usual with several converters in one station whether they arephasedisplaced with respect to each other. Furthermore, the deviceaccording to the invention operates without affecting the regulation ofthe converter in any other respect, which is of importance.

I claim:

1. In a converter arrangement for connection to an alternating currentnetwork (RST), said converter having rectifiers controlled by a controlpulse device (8) ramp-function the form of an oscillator which deliverscontrol pulses (g, g,,) to the individual rectifiers, means formeasuring the delay angle (0:) comprising a ramp-function generator(11-13) for each rectifier phase in the current converter connected tosaid alternating current network and including a timing device (M1)responsive to the passages through zero of the commutation voltages m UU derived from the network for the converter rectifiers, a sensing andholding circuit, a connecting member (S1 S3) connecting saidramp-funtion generator to said sensing and holding circuit (A4, C4),said connecting member being controlled by the control pulse device ofthe current converter, so that the signal (U a stored in said sensingand holding circuit at any moment corresponds to the delay angle for therectifier last ignited.

2. In a converter arrangement as claimed in claim 1, there being foreach ramp-function generator a sensing and holding circuit, thecorresponding connecting member being controlled by correspondingoutputs from the control pulse device of the current converter.

3. In a converter arrangement as claimed in claim 1 having at least twoseries-connected current converters (14-16), each converter beingprovided with such means for measuring the delay angle of the rectifiersand also with its own control system (18, 19), means (A5) to derive theaverage value of the delay angles 3,806,793 6 01 -01 in all theconverters and a comparison device form of an extra signal to thecontrol system of the con- (A6) in each converter to indicate thedeviation of the verter to correct its delay angle so that the delayangles delay angle of that particular converter from said in the variousconverters are of equal magnitude. means value, and means to supply thisdeviation in the

1. In a converter arrangement for connection to an alternating currentnetwork (RST), said converter having rectifiers controlled by a controlpulse device (8) ramp-function the form of an oscillator which deliverscontrol pulses (g1 - g6) to the individual rectifiers, means formeasuring the delay angle ( Alpha ) comprising a ramp-function generator(11-13) for each rectifier phase in the current converter connected tosaid alternating current network and including a timing device (M1)responsive to the passages through zero of the commutation voltages(Utr, Urs, Ust) derived from the network for the converter rectifiers, asensing and holding circuit, a connecting member (S1 - S3) connectingsaid ramp-funtion generator to said sensing and holding circuit (A4,C4), said connecting member being controlled by the control pulse deviceof the current converter, so that the signal (U ) stored in said sensingand holding circuit at any moment corresponds to the delay angle for therectifier last ignited.
 2. In a converter arrangement as claimed inclaim 1, there being for each ramp-function generator a sensing andholding circuit, the corresponding connecting member being controlled bycorresponding outputs from the control pulse device of the currentconverter.
 3. In a converter arrangement as claimed in claim 1 having atleast two series-connected current converters (14-16), each converterbeing provided with such means for measuring the delay angle of therectifiers and also with its own control system (18, 19), means (A5) toderive the average value of the delay angles ( Alpha 1- Alpha 4) in allthe converters and a comparison device (A6) in each converter toindicate the deviation of the delay angle of that particular converterfrom said means value, and means to supply this deviation in the form ofan extra signal to the control system of the converter to correct itsdelay angle so that the delay angles in the various converters are ofequal magnitude.